This work concerns the resolution of normal immunoglobulin light chains by gel isoelectric focusing. In our previous work we showed that normal light chains can be resolved reproducibly into a series of discrete bands by isoelectric focusing. The light chain IF-spectrotype was found to be virtually identical for light chains of different individuals of a species, but clear differences could be detected even in comparisons of closely related species. Comparison of the light chains of various inbred mouse strains led to the discovery of a genetically controlled polymorphism involving several focusing bands. Genetic studies enabled us to show close linkage of the polymorphism with the Ly-2,3 locus on chromosome 6 of the mouse. Our object now is to characterize the focusing bands observed in normal light chains and to define the genetic polymorphism at the molecular level. Our working hypothesis is that the bands observed in the focusing pattern of normal light chains may represent groups of light chains belonging to the same v-region sequence subgroup, as defined in the myeloma amino acid sequence data. We plan to use several approaches to characterize the focusing bands: (1) direct isolation and sequence analysis; (2) immunological identification, using antisera directed at specific subgroups of mouse light chain and (3) systematic comparison of the normal light chain pattern with mouse myeloma proteins. The latter experiments will be in collaboration with Drs. M. Weigert and M. Potter, who have agreed to furnish myeloma material. Preliminary results have shown that the "normal" light chain isoelectric focusing pattern can be closely approximated using reconstituted mixtures of myeloma proteins. If successful, this work will provide for the first time a means of characterizing the content of normal immunoglobulin in terms of definable v-region sequence subgroups. It will thus have potential usefulness in many areas of immunology and medicine where analysis of immunoglobulin is needed.